A. Field of the Invention
The present invention relates to an organic EL display device and a method of driving the device, in particular, to a passive matrix type organic EL display device that exhibits enhanced brightness and reduced power consumption and a method of driving such a device.
B. Description of the Related Art
An organic EL display device performs high visibility owing to the self light emitting nature and low voltage driving ability thereof. Accordingly, it is being actively researched for practical applications. A type of known organic EL light emitting element composing each pixel of an organic EL display device comprises an anode of a transparent conductive film formed on a transparent substrate and an organic layer consisting of a hole transport layer and a light emitting layer (an organic layer of two layer structure). In another known structure, the organic layer consists of three layers: a hole transport layer, a light emitting layer, and an electron transport layer.
The light emitting mechanism of an organic EL light emitting element is considered as follows. An exciton is generated in a fluorescent dye molecule of the light emitting layer with an electron injected from a cathode and a hole injected from an anode. Light emission occurs in a process of irradiating recombination of the exciton. The generated light is emitted through the anode of a transparent conductive film and the transparent substrate.
A passive matrix type (simple matrix type) display device as shown in FIG. 8 is one of the display devices using organic EL light emitting elements. A passive matrix type organic EL display device comprises a plurality of anode elements on a transparent substrate, a plurality of cathode elements perpendicular to the anode elements, and an organic layer including organic light emitting layers sandwiched by these electrode elements. Each pixel is formed at a crossing point of an anode element and a cathode element. A plurality of pixels are arranged to form a display area. The anode and cathode elements are formed extending from the display area to a periphery of the substrate. The extended parts are connection parts connecting to a driver circuit. The connection parts connect to an external driver circuit, to construct an organic EL display device. Research recently has been done on high precision colored passive matrix type organic EL display devices that take advantage of quick response at light emission of an organic EL light emitting device. The organic EL displays are highly expected to achieve high quality display such as full color display and moving image display at a low cost in various application fields of information apparatuses.
As described previously, an organic EL light emitting device is a device utilizing light emission by current injection, and requires a driver circuit that controls a larger current than in electric field-driven devices such as liquid crystal display devices, and an anode and a cathode that allow conduction of such a large current. For electrodes of the passive matrix type organic EL display devices, an anode is made of a transparent conductive metal oxide such as indium tin oxide (ITO), indium lead oxide, or tin oxide, and a cathode is made of a low work function metal such as an aluminum alloy or a magnesium alloy.
Japanese Laid-open Publication No. H9-232074 discloses a technique to reduce power consumption associated with operation of a passive matrix type organic EL display device.
A passive matrix type organic EL display device having X×Y pixels in the display area must drive all pixels in the display area by X+Y electrodes of anodes and cathodes all together. Consequently, the pixels other than the pixels selected in scanning operation by the driver circuit are also influenced by the electric potential of the electrodes (for example, anodes) connecting to the selected pixels.
In a specific case with cathodes of scanning electrode elements of which an electrode element is selected at a moment, and anodes of data electrode elements in the direction crossing the scanning electrode elements, a passive matrix type organic EL display device is operated by a push-pull type driver circuit that changes the connection point of the electrode elements by means of a switching element. In this case, one of the scanning electrode elements (cathodes) is selected and connected to the ground by the switching element. A voltage (forward voltage) for light emission of the organic EL light emitting element is applied by this selected scanning electrode element and a data electrode element (anode) connected to a display current source by a switching element. Scanning electrode elements that are not selected are connected to a bias power supply by switching elements. A reverse bias voltage is applied to the organic EL light emitting element of an unselected scanning electrode element by the unselected scanning electrode element and a data electrode element connected to the ground by a switching element. After a display is accomplished in a selected scanning electrode element, a selected electrode element is switched sequentially. An organic EL light emitting element, having a structure with an organic light emitting layer sandwiched by electrode elements, has a large capacitor component parallel to a diode component. Charging and discharging of the large capacitor component occur due to the forward voltage and the reverse bias voltage at every time of switching of a selected scanning electrode element.
The charging and discharging are described more in detail below. In a passive matrix type organic EL display device in a display operation, one scanning electrode element is selected for a certain period and the other scanning electrode elements are not selected in this period. Almost throughout the period, the organic EL light emitting elements driven by unselected scanning electrode elements are subjected to a reverse bias voltage. This is because the switching elements are controlled to set the data electrode element at the ground potential, the selected scanning electrode element at the ground potential, and the unselected scanning electrode elements at the potential of the power supply. In this period, the data electrode element is connected to the potential of the power supply to light the organic EL light emitting element and light emitting current flows in the organic EL light emitting element connecting to the selected scanning electrode element. At this time, the capacitor component of the organic EL light emitting element is charged, and at the same time, the organic EL light emitting element connecting to an unselected scanning electrode element is also charged by the reverse bias voltage. As a result, a problem arises that sufficient charges cannot be supplied to the organic EL light emitting element to be lighted. If the driver circuit for supplying charges to anode elements is a constant current type, the charging process takes more time and the desired brightness can not be attained during that transient period, thus, average brightness is decreased. Accordingly, a magnitude of the constant current is set at a higher level to ensure a desired average brightness. The organic EL light emitting element suffers degradation in electric current efficiency, an increase in power consumption, and a shortening of operation life. In addition, the power loss due to charging and discharging on every switching of selected scanning electrode element cannot be ignored.
To solve this problem, Japanese Unexamined Patent Application Publication No. H9-232074 discloses a method of cathode reset. In the process of switching the selected scanning electrode element (cathode element) to the next, at first, every scanning electrode element is once connected to the power supply at the ground potential. Thereby, the subsequently selected scanning electrode element receives charges through other scanning electrode elements, accumulating charges in some amount before lighting. In the method of cathode reset, however, a large inrush current flows into the lighting organic EL light emitting element from the unselected scanning electrode elements all at once, which raises the problem of a heavy load on the driver IC. Further in the method of cathode reset, the power source potential of the scanning electrode elements must be set lower than the power source potential of the data electrode anode elements, and avoid light emission in the pixels.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.